Glutamate antagonist therapy reduces neurologic deficits produced by focal central nervous system ischemia

NMDA receptor-mediated Ca2+ signaling: Impact on cell cycle regulation and the development of neurodegenerative diseases and cancer

NMDA receptors are Ca2+-permeable ligand-gated ion channels that mediate fast excitatory transmission in the central nervous system. NMDA receptors regulate the proliferation and differentiation of neural progenitor cells and also play critical roles in neural plasticity, memory, and learning. In addition to their physiological role, NMDA receptors are also involved in glutamate-mediated excitotoxicity, which results from excessive glutamate stimulation, leading to Ca2+ overload, and ultimately to neuronal death. Thus, NMDA receptor-mediated excitotoxicity has been linked to several neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's, dementia, and stroke. Interestingly, in addition to its effects on cell death, aberrant expression or activation of NMDA receptors is also involved in pathological cellular proliferation, and is implicated in the invasion and proliferation of various types of cancer. These disorders are thought to be related to the contribution of NMDA receptors to cell proliferation and cell death through cell cycle modulation. This review aims to discuss the evidence implicating NMDA receptor activity in cell cycle regulation and the link between aberrant NMDA receptor activity and the development of neurodegenerative diseases and cancer due to cell cycle dysregulation. The information presented here will provide insights into the signaling pathways and the contribution of NMDA receptors to these diseases, and suggests that NMDA receptors are promising targets for the prevention and treatment of these diseases, which are leading causes of death and disability worldwide.

Excitotoxicity: Still Hammering the Ischemic Brain in 2020

Interest in excitotoxicity expanded following its implication in the pathogenesis of ischemic brain injury in the 1980s, but waned subsequent to the failure of N-methyl-D-aspartate (NMDA) antagonists in high profile clinical stroke trials. Nonetheless there has been steady progress in elucidating underlying mechanisms. This review will outline the historical path to current understandings of excitotoxicity in the ischemic brain, and suggest that this knowledge should be leveraged now to develop neuroprotective treatments for stroke.

Analytic Reviews : Potential New Therapies for Acute Ischemic Stroke

Evidence from animal models of acute stroke suggests ischemia may be reversible if blood flow is restored in the first few hours. Studies of human stroke using posi tron emission tomography demonstrate areas with re duced blood flow and relatively preserved metabolism, indicating potentially reversible ischemic brain. Resto ration of blood flow during this reversible phase should improve outcome after stroke.

Many therapeutic strategies for treatment of acute ischemic stroke have been proposed, including increas ing collateral flow, removing vascular obstructions, and interfering with the intracellular cascade of events that lead to neuronal cell death. Hypervolemic hemodilution reduces viscosity and increases cerebral blood flow, and this may hopefully raise blood flow above the critical threshold of irreversible ischemia. Naloxone, calcium channel blockers, and glutamate antagonists alter blood flow and influence intracellular events during and after acute ischemia. Thrombolytic therapy restores blood flow by lysis of obstructing clot. These therapies show promise in preliminary studies, but additional ran domized controlled studies are needed.

REVIEW ■ : Neuroprotection in Brain Ischemia: An Update (Part I

Ischemic brain injury produced by stroke or cardiac arrest is a major cause of human neurological disability. Steady advances in the neurosciences have elucidated pathophysiological mechanisms of brain ischemia and have suggested many therapeutic approaches directed at specific injury mechanisms to achieve neuroprotection of the acutely ischemic brain. The first portion of this two-part review highlights the differentiating features and pathological mechanisms of focal and global cerebral ischemic injury and summarizes a wealth of recent evidence as to how antagonism of excitatory amino acid neurotoxicity, mediated via NMDA as well as non-NMDA receptors, may offer a means of diminishing the extent of ischemic injury. The Neuroscientist 1:95-103, 1995

Energy restriction negates NMDA receptor antagonist efficacy in ischemic stroke

Preclinical evaluation of drugs for neurological disorders is usually performed on overfed rodents, without consideration of how metabolic state might affect drug efficacy. Using a widely employed mouse model of focal ischemic stroke, we found that that the NMDA receptor antagonist dizocilpine (MK-801) reduces brain damage and improves functional outcome in mice on the usual ad libitum diet, but exhibits little or no therapeutic efficacy in mice maintained on an energy-restricted diet. Thus, NMDA receptor activation plays a central role in the mechanism by which a high dietary energy intake exacerbates ischemic brain injury. These findings suggest that inclusion of subjects with a wide range of energy intakes in clinical trials for stroke may mask a drug benefit in the overfed/obese subpopulation of subjects.

Anti-apoptotic and neuroprotective effects of α-lipoic acid on spinal cord ischemia-reperfusion injury in rabbits

PURPOSE

Radical oxygen species produced after injury counteracts antioxidant activity and frequently causes severe oxidative stress for the tissues. Alpha-lipoic acid is a powerful metabolic antioxidant with immunomodulatory effects which provides neuroprotection. The aim of this study is to investigate the neuroprotective and anti-apoptotic effects of alpha-lipoic acid on spinal cord ischemia-reperfusion.

METHODS

Twenty-four adult, male, New Zealand rabbits were divided into sham (n = 8), control (n = 8), and treatment groups (n = 8). The abdominal aorta was clamped for 30 min by an aneurysm clip, approximately 1 cm below the renal artery and 1 cm above the iliac bifurcation in control and treatment groups. Only laparotomy was performed in the sham group. Twenty-five cubic centimeters of saline in control group and 100 mg/kg lipoic acid were administered intraperitoneally in the treatment group after closure of the incision. The animals were killed 48 h later. Spinal cord segments between L2 and S1 were harvested for analysis. Levels of nitric oxide, glutathione, malondialdehyde, advanced oxidation protein products, and superoxide dismutase were analyzed as markers of oxidative stress and inflammation. Caspase-3 activity was analyzed to detect the effect of lipoic acid on apoptosis.

RESULTS

In all measured parameters of oxidative stress, administration of lipoic acid significantly demonstrated favorable effects. Both plasma and tissue levels of nitric oxide, glutathione, malondialdehyde, and advanced oxidation protein products significantly changed in favor of antioxidant activity. There was no significant difference between the plasma superoxide dismutase levels of the groups. Histopathological evaluation of the tissues also demonstrated significant decrease in cellular degeneration and infiltration parameters after lipoic acid administration. However, lipoic acid has no effect on caspase-3 activity.

CONCLUSIONS

Although further studies considering different dose regimens and time intervals are required, the results of the present study prove that alpha-lipoic acid has favorable effects on experimental spinal cord ischemia-reperfusion injury.

Lactacidosis modulates glutathione metabolism and oxidative glutamate toxicity

Lactate and acidosis increase infarct size in humans and in animal models of cerebral ischemia but the mechanisms by which they exert their neurotoxic effects are poorly understood. Oxidative glutamate toxicity is a form of nerve cell death, wherein glutamate inhibits cystine uptake via the cystine/glutamate antiporter system leading to glutathione depletion, accumulation of reactive oxygen species and, ultimately, programmed cell death. Using the hippocampal cell line, HT22, we show that lactate and acidosis exacerbate oxidative glutamate toxicity and further decrease glutathione levels. Acidosis but not lactate inhibits system , whereas both acidosis and lactate inhibit the enzymatic steps of glutathione synthesis downstream of cystine uptake. In contrast, when glutathione synthesis is completely inhibited by cystine-free medium, acidosis partially protects against glutathione depletion and cell death. Both effects of acidosis are also present in primary neuronal and astrocyte cultures. Furthermore, we show that some neuroprotective compounds are much less effective in the presence of lactacidosis. Our findings indicate that lactacidosis modulates glutathione metabolism and neuronal cell death. Furthermore, lactacidosis may interfere with the action of some neuroprotective drugs rendering these less likely to be therapeutically effective in cerebral ischemia.

Potential cardiovascular applications of glutamate, aspartate, and other amino acids

Cardioplegic solutions rich in the hydrophilic, basic amino acids, glutamate and aspartate, have enhanced myocardial preservation and left ventricular function. This has been demonstrated in assorted animal preparations involving ischemia with and without reperfusion. Published clinical data, though limited, strongly support the contention that these amino acids have myocardial protective properties. Several biochemical mechanisms exist by which certain amino acids may attenuate ischemic or reperfusion injury. Glutamate and aspartate may become preferred myocardial fuels in the setting of ischemia. They may also reduce myocardial ammonia production and reduce cytoplasmic lactate levels, thereby deinhibiting glycolysis. Some amino acids may become substrate for the citric acid cycle. Glutamate and aspartate also move reducing equivalents from cytoplasm to mitochondria where they are necessary for oxidative phosphorylation and energy generation. A rationale exists for the use of an amino acid-rich cardioplegia-like solution in myocardial infarction. These solutions are safe and inexpensive.

The phenylpropanoid micronutrient chlorogenic acid improves clinical rating scores in rabbits following multiple infarct ischemic strokes: Synergism with tissue plasminogen activator

The present study assessed whether chlorogenic acid (CGA), a phenylpropanoid molecule that has multiple mechanisms of action would be useful to attenuate behavioral deficits associated with embolic strokes using the rabbit small clot embolic stroke model (RSCEM). Quantal analysis for each treatment determines the quantity of microclots (mg) that produce neurologic dysfunction in 50% of a group of animals (P(50)), with intervention considered beneficial if it increases the P(50) compared to controls. CGA (50 mg/kg) injected 5 min post-embolization significantly increased behavioral function and the P(50) to 3.61+/-0.52 mg (n=19) compared to 1.58+/-0.15 mg (n=26) in controls. In addition, CGA also increased the P(50) to 2.57+/-0.28 mg (n=18) when administered 1 h post-embolization, but was ineffective when given 3 h following embolization (P(50)=1.22+/-0.24 mg, n=18). For combination studies with the thrombolytic tissue plasminogen activator (tPA), we used tPA at a standard dose of 3.3 mg/kg, which significantly increased the P(50) to 2.89+/-0.29 mg (n=17) when administered 1 h after embolization, but not 3 h after embolization (P(50)=1.54+/-0.27 mg, n=18). However, when tPA (3.3 mg/kg) was combined with CGA (50 mg/kg) and administered 3 h following embolization, there was a significant increase in behavioral function as evidenced by an increase in the P(50) value to 3.40+/-0.76 mg (n=23). In conclusion, as a mono-therapy CGA effectively reduced behavioral deficits when given up to 1 h following embolic strokes in rabbits. Moreover, there was a synergistic effect of the combination of tPA with CGA when administered 3 h following embolization. The results show that the therapeutic window for a standard effective dose of tPA could be increased by administration of CGA, suggesting that it may be most useful as a co-therapy with a standard thrombolytic treatment regimen.

Addition of intravenous N-methyl-D-aspartate receptor antagonists to local fibrinolytic therapy for the optimal treatment of experimental intracerebral hemorrhages

OBJECT

Fibrinolytic therapy with recombinant tissue plasminogen activator (rtPA) is considered a treatment option in patients with deep-seated intracerebral hemorrhage (ICH). Nevertheless, the results of animal experiments have shown that tPA exerts pleiotropic actions in the brain, including regulation of vasoactivity, amplification of calcium conductance by cleavage of the N-methyl-D-aspartate (NMDA) receptor subunit, and activation of metalloproteinases, which increase excitotoxicity, damage the blood-brain barrier, and worsen edema. The authors investigated whether the noncompetitive NMDA receptor antagonist MK801 can be used as an adjuvant therapy in combination with rtPA to attenuate the unfavorable delayed edema formation and inflammation observed following rtPA therapy in an experimental porcine model of ICH.

METHODS

Twenty pigs were used in this study; MK801 (0.3 mg/kg) was administered to each pig intravenously immediately after hematoma induction and on the 1st and 3rd day after hematoma induction. Ten of the 20 pigs were randomly assigned to fibrinolytic therapy with rtPA (MK801-tPA group), whereas in the remaining 10 control animals (MK801 group) the hematomas were allowed to follow their natural courses of resorption. The extent of edema formation was evaluated using magnetic resonance (MR) imaging volumetry on Days 0, 4, and 10 after hematoma induction and was compared with histopathological changes found at necropsy. The mean edema volumes in these two groups were also compared with that in the group of nine pigs examined in a preceding experimental series, in which the animals' hematomas were only treated with rtPA (tPA group). In the 10 animals in the MK801-tPA group, the mean perihematoma edema volume on MR images had not significantly increased by Day 4 (p < 0.08) or Day 10 (p < 0.35) after hematoma induction. In the 10 animals in the MK801 group, the increase in mean perifocal edema size was significant after 4 days (p < 0.001) and nonsignificant after 10 days (p < 0.09). In the nine animals in the tPA group, the mean edema volume significantly increased by Days 4 (p < 0.002) and 10 (p < 0.03).

CONCLUSIONS

As suggested by the reduction in delayed edema volume and the inflammatory response, MK801 modifies the neurotoxic properties of rtPA but not those of blood degradation products. Possibly, fibrinolytic therapy of ICH is more beneficial if combined with agents such as MK801.

Delta9-tetrahydrocannabinol protects hippocampal neurons from excitotoxicity

Excitotoxic neuronal death underlies many neurodegenerative disorders. Because cannabinoid receptor agonists act presynaptically to inhibit glutamate release, we examined the effects of Win 55212-2, a full agonist at CB(1) receptors, and Delta(9)-tetrahydrocannabinol (THC), a partial agonist, on the survival of neurons exposed to an excitotoxic pattern of synaptic activity. Reducing the extracellular Mg(2+) concentration ([Mg(2+)](o)) to 0.1 mM evoked an aberrant pattern of glutamatergic activity that produced synaptically mediated death of rat hippocampal neurons in culture. Neuronal viability was quantified with a multiwell fluorescence plate scanner equipped to detect propidium iodide fluorescence. Win 55212-2 (100 nM) and THC (100 nM) significantly reduced 0.1 mM [Mg(2+)](o)-induced cell death by 77 +/- 11% and 84 +/- 8%, respectively. Interestingly, the protection afforded by THC was not significantly different from that produced by Win 55212-2, suggesting that attenuation without a complete block of excitatory activity is sufficient for neuroprotection. The effect of prolonged drug exposure on the neuroprotection afforded by cannabinoid receptor agonists was also studied. When cultures were pretreated for 24 h with Win 55212-2 (100 nM) or THC (100 nM), inhibition of 0.1 mM [Mg(2+)](o)-induced toxicity was significantly reduced to 39 +/- 19% and 45 +/- 13%, respectively. Thus, desensitization of CB(1) receptors diminishes the neuroprotective effects of cannabinoids. This study demonstrates the importance of agonist efficacy and the duration of treatment on the neuroprotective effects of cannabinoids. It will be important to consider these effects on neuronal survival when evaluating pharmacologic treatments that modulate the endocannabinoid system.

Memantine, an uncompetitive low affinity NMDA open-channel antagonist improves clinical rating scores in a multiple infarct embolic stroke model in rabbits

The blockade of NMDA receptors has been pursued as a strategy to reduce the consequences of acute ischemic stroke (AIS) and NMDA receptors remain a valid therapeutic target to treat AIS. Because the pharmacological and toxicity profile of memantine in Alzheimer's disease patients appears to be good, we determined whether memantine would be effective at improving behavioral performance following embolic strokes in rabbits. For these studies, we used a rabbit multiple infarct ischemia model with a well-defined behavioral endpoint. In this study, memantine dissolved in PBS was given intravenously either as a bolus injection (over 1 min) or infused over 60 min. The P(50) of the control groups measured 24 h after embolization were 1.12 +/- 0.18 mg and 1.08 +/- 0.23 mg for the bolus injected and infused groups, respectively. Bolus injections of memantine at 1 mg/kg and 10 mg/kg were not effective at altering the P(50) value and memantine at a dose of 25 mg/kg was lethal. However, slowly infused memantine (25 mg/kg) significantly increased the P(50) value to 2.31 +/- 0.48 mg and 3.13 +/- 1.13 mg when given 5 and 60 min following embolization, respectively. Memantine administered 180 min following embolization also increased the P(50) value to 2.69 +/- 2.21 mg, but the response was variable. These results suggest that uncompetitive NMDA antagonists, more specifically open channel blockers, which may be alternatives to high affinity NMDA antagonists, may have substantial therapeutic benefit for the treatment of AIS and memantine or new dual activity analogs of memantine should be further pursued as a useful therapy to treat the behavioral deficits associated with multiple-infarct ischemia.

5-Bromo-2'-deoxyuridine is selectively toxic to neuronal precursors in vitro

The effect of 5-bromo-2'-deoxyuridine (BrdU) incorporation on the phenotype of progeny derived from expanded E18 rat striatal precursors was examined. BrdU was administered to cultures for 24 h prior to differentiation. Results revealed that there was selective toxicity of this compound to developing TuJ1+ neurons, but not glia, at concentrations used in most labelling studies. Therefore, a BrdU dose-response curve from 0.2 microM to 10 microM was established. The optimum dose of BrdU for labelling cells was 0.2 microM, well below the 1-10 microm recommended concentration. This dose resulted in the survival of significantly more newborn BrdU/TuJ1+ double-labelled neurons and eliminated the toxic effects of BrdU. Administration of 10 microm BrdU resulted in a significant decrease in extracellular regulated kinase (ERK) phosphorylation compared with untreated cultures, this could be completely restored by the administration of either N-methyl-D-aspartate (NMDA) receptor antagonists such as MK801 or the nitric oxide synthesis inhibitor L-methyl-arginine methyl ester (L-NAME). Our results show that high levels of BrdU are selectively toxic to neurons through a mechanism that activates classical cell death pathways. This has implications for labelling studies both in vivo and in vitro.

Ischemic tolerance in chemical preconditioning: Possible role of astrocytic glutamine synthetase buffering glutamate-mediated neurotoxicity

Glutamine synthetase (GS), localized to astrocyte is a key enzyme in the glutamate-glutamine pathway in the brain. 3-Nitropropionic acid (3-NPA) is an irreversible inhibitor of succinate dehydrogenase in the tricarboxylic-acid cycle, and provides ischemic tolerance to the brain. So far, there have been no reports on the relationship of astrocytic GS and ischemic tolerance by chemical preconditioning. In order to test the hypothesis that astrocytes serve a pivotal role in 3-NPA-induced chemical preconditioning, we have investigated the temporal profile of GS expression in astrocyte parallel with those of glial fibrillary acidic protein and heat-shock protein 70 (HSP70). In our rat model of permanent focal ischemia, preconditioning with 3-NPA singnificantly reduced the subsequent neurological deficits and infarct volume within 24-72 hours after treatment. Immunohistochemically, protoplasmic astrocytes in the cortex and striatum were activated in terms of upregulation of GS and more abundant protoplasmic processes with 3-NPA preconditioning, however, HSP70 expression could not be induced. Thus, the activation of astrocytes and upregulation of GS play an important role in 3-NPA-induced preconditioning but HSP70 does not. In view of glutamate being imposed on the cerebral ischemic damage, the astrocytic GS may contribute to 3-NPA-induced ischemic tolerance.

3alpha-OL-5-beta-pregnan-20-one hemisuccinate, a steroidal low-affinity NMDA receptor antagonist improves clinical rating scores in a rabbit multiple infarct ischemia model: synergism with tissue plasminogen activator

We previously showed that the neuroactive steroid 3alpha-ol-5-beta-pregnan-20-one hemisuccinate (ABHS), a low-affinity NMDA receptor antagonist, improves behavior in rabbits following spinal cord ischemia. The present study assessed whether this also occurs following cerebral ischemia. We used the rabbit small clot embolic stroke model (RSCEM), a model of multiple infarct ischemia, which has a well-defined behavioral endpoint. Quantal analysis for each treatment determines microclots (mg) that produce neurologic dysfunction in 50% of a group of animals (P(50)), with treatment considered beneficial if it increases the P(50) compared to controls. ABHS (25 mg/kg) injected 5 min post-embolization significantly (P = 0.046) increased the P(50) to 2.60 +/- 0.69 mg compared to 1.15 +/- 0.19 mg in controls but was ineffective at longer intervals. For combination studies with tissue plasminogen activator (tPA), a cumulative control curve (P(50) = 1.18 +/- 0.25 mg) was used for statistical comparisons. Low-dose tPA (0.9 mg/kg) and ABHS, given 60 min post-embolization, improved behavior synergistically (P(50) = 2.44 +/- 0.44 mg), compared to tPA (P(50) = 1.25 +/- 0.25 mg) or ABHS (P(50) = 1.05 +/- 0.24 mg) alone. A standard tPA dose (3.3 mg/kg) significantly increased the P(50) at 60 (2.69 +/- 0.19 mg) but not 180 min (1.28 +/- 0.31 mg) post-embolization. However, when combined with ABHS, an effect was evident even 180 min post-embolization (P(50) = 2.31 +/- 0.48 mg). Although the therapeutic window for ABHS is limited, it significantly enhanced neuroprotection by low-dose tPA and increased the therapeutic window for tPA, suggesting that it may be most useful as a co-therapy with thrombolytics for stroke.

MK-801 improves neurological and histological outcomes after spinal cord ischemia induced by transient aortic cross-clipping in rats

BACKGROUND

Glutamergic excitotoxicity has been shown to play a deleterious role in the pathophysiology of ischemic spinal cord injury (ISCI). The aim of this study was to investigate the neuroprotective effect of a single dose of MK-801, an antiexcitotoxic drug, in a rat model of ISCI.

METHODS

Ischemic spinal cord injury was induced for 17 minutes in Sprague-Dawley rats using direct aortic arch, just proximal to the left common carotid artery, plus left subclavian artery cross-clamping through a left-sided limited thoracotomy. Study groups were as follows: control group (n = 8) receiving only vehicle and experimental group (n = 8) receiving a single dose of MK-801 (1 mg/kg IV) 10 minutes before aortic clamping. Neurological examination was performed at 6 hours, 24 hours, and daily up to 96 hours. Rats were sacrifice at methylprenisolone socium succinate 96 hours, and spinal cords were removed for histopathology.

RESULTS

All the control rats had severe permanent neurological deficits after ISCI, whereas the MK-801-treated rats had statistically (P < .05) better neurological outcome and good recovery. Histopathology revealed severe neuronal necrosis in the lumbar gray matter of control rats, whereas MK-801-treated rats showed mild injury.

CONCLUSION

These results demonstrate that combined temporary clipping of the aortic arch (just proximal to the left common carotid artery) plus left subclavian artery for 17 minutes reproduces reliable paraplegia, and a single dose of MK-801 given before ISCI provides significant neuroprotection.

Enhanced striatal NR2B-containing N-methyl-D-aspartate receptor-mediated synaptic currents in a mouse model of Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disease caused by expansion of a polyglutamine tract near the N terminus of the protein huntingtin, leading to dramatic loss of striatal medium-sized spiny GABAergic projection neurons (MSNs). Evidence suggests overactivation of N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs) contributes to selective degeneration of MSNs in HD. Striatal MSNs are enriched in NR2B, and whole cell current and excitotoxicity mediated predominantly by the NR2B subtype of NMDARs is increased with expression of mutant huntingtin in transfected cell lines and striatal MSNs from mice models. To test whether synaptic NMDAR current is altered by mutant huntingtin expression, we recorded striatal MSN excitatory postsynaptic currents (EPSCs) evoked by stimulation of cortical afferents in corticostriatal slices from YAC72 mice and their wild-type (WT) littermates at age 21-31 days. The ratio of NMDAR- to AMPAR-mediated EPSC amplitude was significantly increased in YAC72 compared to WT mice. Furthermore, using a paired-pulse stimulation protocol as a measure of presynaptic glutamate release probability, we found no significant differences between YAC72 and WT striatal MSN responses. These data suggest selective potentiation of postsynaptic NMDAR activity at corticostriatal synapses in YAC72 mice. Measurements of EPSC decay kinetics, as well as the effects of NR2B-subtype selective antagonists and glycine concentration on EPSC amplitude, are consistent with the majority of postsynaptic NMDARs being triheteromers of NR1/NR2A/NR2B in both WT and YAC72 mice. Together with previous results, our data suggest that enhanced activity of NR2B-containing NMDARs is one of the earliest changes leading to neuronal degeneration in HD.

The neuroactive steroid 3-alpha-ol-5-beta-pregnan-20-one hemisuccinate, a selective NMDA receptor antagonist improves behavioral performance following spinal cord ischemia

The initial response to an ischemic event is the rapid release of excitatory amino acid's followed by the activation of the "ischemic cascade". It has been suggested that neurosteroids, which act as negative modulators of excitatory amino acid receptors, may improve behavioral functions and promote neuronal survival following ischemia. The present study evaluated the pharmacological effects of 3-alpha-ol-5-beta-pregnan-20-one hemisuccinate (ABHS), a neurosteroid that inhibits excitatory amino acid receptor function, in a rabbit reversible spinal cord ischemia model (RSCIM). ABHS was administered (25 mg/kg) intravenously (i.v.) 5 or 30 min following the start of occlusion to groups of rabbits exposed to ischemia induced by temporary occlusion of the infrarenal aorta. The group P50 represents the duration of ischemia (min) associated with a 50% probability of resultant permanent paraplegia. Quantal analysis indicated that the P50 of the control group was 23.44 +/- 4.32 min. Using the RSCIM, neuroprotection is observed if a drug significantly prolongs the P50 compared to the control group. Treatment with ABHS (25 mg/kg) 5 min post-occlusion significantly (p < 0.05) prolonged the P50 of the group to 49.18 +/- 10.44 min, an increase of 110%. The effect of ABHS was not durable following a single injection since a significant difference between the control and ABHS-treated groups was not measurable at 48 h. However, if ABHS was injected 5 min following the start of ischemia and again 24 h after ischemia, there was a persistent effect of the drug at 48 h. Moreover, ABHS also increased the tolerance to ischemia if administered 30 min following the start of occlusion. Our results suggest that neuroactive steroids such as ABHS, which are selective NMDA receptor antagonists, may have substantial therapeutic benefit for the treatment of ischemic injuries including spinal cord neurodegeneration and stroke.

Protective effect of melatonin on experimental spinal cord ischemia

STUDY DESIGN

Experimental animal model to assess ischemic spinal cord injury following occlusion of the thoraco-abdominal aorta.

OBJECTIVES

To measure whether melatonin administered to rabbits before and after occlusion exerts an effect on the repair of ischemia-reperfusion (IR) injury.

SETTING

Medical Biology Laboratory, Inonu University, Malatya, Turkey.

METHODS

Rabbits were divided into three IR treatment groups and one sham-operated (ShOp) control group. The three treatment groups had their infrarenal aorta temporarily occluded for 25 min, while the ShOp group had laparotomy without aortic occlusion. Melatonin was administered either 10 min before aortic occlusion or 10 min after the clamp was removed. Physiologic saline was administered to the control animals. After treatment, the animals were euthanized and lumbosacral spinal cord tissue was removed for the determination of relevant enzyme activities.

RESULTS

Malondialdehyde levels, indicating the extent of lipid peroxidation, were found to be significantly increased in the nonmelatonin treated (IR) group when compared to the ShOp group. Melatonin, whether given to pre- or post occlusion groups, suppressed malondialdehyde levels below that of the ShOp group. Catalase (CAT) and glutathione peroxidase (GSH-Px) enzyme activities were increased in the IR group compared to the ShOp group. Melatonin given preocclusion resulted in a significant decrease in both CAT and GSH-Px enzyme levels. The superoxide dismutase (SOD) enzyme activity was decreased in the ischemia-reperfusion treatment group. However, the melatonin treatment increased SOD enzyme activity to levels approximating that of the ShOp group.

CONCLUSION

To our knowledge, this is the first study that shows the effects of melatonin administered both pre- and postischemia on induced oxidative damage to injured spinal cords. Our data also expands on reports that melatonin administration may significantly reduce the incidence of spinal cord injury following temporary aortic occlusion.

Perinatal brain damage--from pathophysiology to prevention

Children undergoing perinatal brain injury often suffer from the dramatic consequences of this misfortune for the rest of their lives. Despite the severe clinical and socio-economic significance, no effective clinical strategies have yet been developed to counteract this condition. This review describes the pathophysiological mechanisms that are implicated in perinatal brain injury. These include the acute breakdown of neuronal membrane potential followed by the release of excitatory amino acids such as glutamate and aspartate. Glutamate binds to postsynaptically located glutamate receptors that regulate calcium channels. The resulting calcium influx activates proteases, lipases and endonucleases which in turn destroy the cellular skeleton. The acute lack of cellular energy during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to preischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. A second wave of neuronal cell damage occurs during the reperfusion phase induced by the postischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Clinical studies have shown that intrauterine infection increases the risk of periventricular white matter damage especially in the immature fetus. This damage may be mediated by cardiovascular effects of endotoxins leading to cerebral hypoperfusion and by activation of apoptotic pathways in oligodendrocyte progenitors through the release of pro-inflammatory cytokines. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies which have been shown to be neuroprotective in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of postischemic induction of mild cerebral hypothermia, the application of the calcium-antagonist flunarizine and the administration of magnesium.

Neuroprotective effects of the spin trap agent disodium-[(tert-butylimino)methyl]benzene-1,3-disulfonate N-oxide (generic NXY-059) in a rabbit small clot embolic stroke model: combination studies with the thrombolytic tissue plasminogen activator

BACKGROUND AND PURPOSE

It has been proposed that the novel spin trap agent disodium-[(tert-butylimino)methyl]benzene-1,3-disulfonate N-oxide (NXY-059) may be useful in the treatment of ischemic stroke. However, there is little information concerning the neuroprotective properties of NXY-059 when administered after an embolic stroke. Moreover, there is no information available concerning the combination of NXY-059 with the only Food and Drug Administration-approved pharmacological agent for the treatment of acute stroke, the thrombolytic tissue plasminogen activator (tPA). Thus, we determined the effects of NXY-059G, a generic form of NXY-059, on behavioral outcome after an embolic stroke when administered alone or in combination with tPA.

METHODS

Male New Zealand White rabbits were embolized by injecting a suspension of small blood clots into cerebral circulation via a carotid catheter. NXY-059G (100 mg/kg) was infused intravenously 5 minutes or 3 hours after embolization, whereas control rabbits received infusions of the saline vehicle. In tPA studies, the thrombolytic was administered intravenously starting 60 minutes or 3 hours after embolization (3.3 mg/kg). In combination studies, NXY-059G was given 5 minutes after embolization, followed by the administration of tPA beginning either 60 minutes or 3 hours after embolization. Behavioral analysis was conducted 24 hours after embolization.

RESULTS

In the vehicle control group, the ES50 value (calculated as the amount of microclots [milligrams] that produce neurological dysfunction [impairment] in 50% of the rabbits within a specific treatment group) measured 24 hours after embolism was 1.04+/-0.31 mg, and this was increased by 153% to 2.54+/-0.72 mg if NXY-059G was administered beginning 5 minutes after embolization. However, if NXY-059G was administered beginning 3 hours after embolization, the ES50 was 2.01+/-0.40 mg. The rabbits treated with tPA alone had an ES50 of 2.64+/-0.66 or 0.63+/-0.35 mg if tPA administration started 60 minutes or 3 hours after embolization, respectively. If tPA was administered after NXY-059G (started at 5 minutes), the ES50 values were 3.15+/-0.50 or 2.66+/-0.82 if tPA administration started 60 minutes or 3 hours after embolization, respectively.

CONCLUSIONS

This study suggests that NXY-059G is neuroprotective and can increase behavioral ratings if administered early after an embolic stroke. In addition, the study shows that NXY-059G can be used in combination with tPA without negative side effects. The drug combination can improve behavioral function and increase ES50 values. However, during the short time course of the behavioral analysis, the combination was not statistically better than either drug alone.

Reduction of ischemic spinal cord injury by dextrorphan: comparison of several methods of administration

OBJECTIVES

We investigated the effect of dextrorphan, an N -methyl-D -aspartate receptor antagonist, on the reduction of ischemic spinal cord injury and the safe clamping time after various methods of administration.

METHODS

Spinal cord ischemia was induced in New Zealand White rabbits by infrarenal aortic clamping and animals were divided into 5 groups. Group A (n = 15) received simple clamping. Groups B (n = 20) and C (n = 35) received dextrorphan pretreatment (10 mg/kg), followed by continuous intravenous or intra-aortic infusion (1 mg/min), respectively. Group D (n = 25) received the same dextrorphan pretreatment and bolus intra-aortic injection at clamping (1 mg per minute of clamping time). Group E (n = 15) received bolus intrathecal injection of dextrorphan (0.2 mg/kg). Each dextrorphan-treated group had a small group of control animals (n = 5). The neurologic status was assessed by the Johnson score (5 = normal, 0 = paraplegic) 48 hours after unclamping, and animals were put to death for histopathologic examination.

RESULTS

All dextrorphan-treated groups showed better neurologic function than the respective control animals (P <.001 vs groups B, C, and D; P =.014 vs group E). The order of efficacy of dextrorphan (as revealed by the average of neurologic status) was as follows: group C > group D (P =.017, after 50 minutes of clamping), group D > group B (P =.014, after 45 minutes of clamping), and group B > group E (P <.001, after 40 minutes of clamping). Histopathologic findings did not necessarily correspond with hind-limb neurologic function.

CONCLUSIONS

Dextrorphan reduced the physical findings associated with ischemic spinal cord injury, and continuous intra-aortic infusion prolonged the safe clamping time significantly more than delivery by other routes.

Influence of posttraumatic hypoxia on behavioral recovery and histopathological outcome following moderate spinal cord injury in rats

Pulmonary dysfunction leading to secondary hypoxia is a common complication of spinal cord injury (SCI). The purpose of this study was to clarify the behavioral and histopathological consequences of posttraumatic hypoxia in an established model of traumatic SCI. Forty-five female Sprague-Dawley rats were randomly assigned to one of four groups, including (1) laminectomy and normoxia (n = 10), (2) laminectomy and hypoxia (n = 11), (3) NYU weight-drop and normoxia (n = 12), and (4) NYU weight-drop and hypoxia (n = 11). For these studies, a moderate injury was induced by adjusting the height of the weight drop (10 g) to 12.5 mm above the exposed spinal cord (T10). Immediately after injury, PaO2 in the hypoxic rats was kept between 30 and 35 mm Hg for 30 min. PaO2 in the normoxic group was maintained over 100 mm Hg, while PaCO2 in all rats was maintained at 35-40 mm Hg. The behavior of the rats was checked every 7 days using the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale. Rats were sacrificed at 8 weeks for quantitative histopathological analysis of lesion areas. During the hypoxic insults, the mean arterial blood pressure dropped in both sham control and weight-drop rats (p < 0.01). At the end of the 8-week monitoring period, BBB scores were 12.5 +/- 3.1 (mean +/- SEM) and 14.2 +/- 3.4 in the normoxic and hypoxic traumatized rats, respectively. No significant difference between the traumatized groups was documented with BBB monitoring. In contrast, the percent of gray matter necrosis at the impact epicenter was significantly increased in hypoxic versus normoxic SCI rats (p < 0.01). These data demonstrate that posttraumatic hypoxia complicated by mild hypotension aggravates the histopathological consequences of SCI and further emphasize the need to control for secondary hypoxic insults after experimental and clinical SCI. Potential explanations for the lack of a correlation between the behavioral and histopathological findings are discussed.

Transient down-regulation of NMDA receptor subunit gene expression in the rat retina following NMDA-induced neurotoxicity is attenuated in the presence of the non-competitive NMDA receptor antagonist MK-801

Excitotoxic challenge has been thought to directly target NMDA-receptive neurons to undergo cell death. Recent evidence suggests that NMDA induced cell death is a selective process and that the specificity may be determined by the subunit composition of the NMDA receptor. Using a rat retinal model, we examined the effects of NMDA induced neurotoxicity on the regulation of NMDA receptor subunit gene and protein expression levels to determine if excitotoxic challenge preferentially regulates one or more of the NMDA receptor subunits. Following NMDA insult, the mRNA levels for NR1(com ), NR2A, NR2B and, to a lesser extent, the NR2C subunit were substantially reduced within 24 hr post-treatment (PT), and remained depressed for up to 48 hr. Levels for NR2D, although initially suppressed as early as 6 hr-PT, were least affected by NMDA insult and showed almost full recovery by 48 hr. By 10 days, the levels of gene expression for all five subunits recovered to levels that were indistinguishable from sham treated and untreated retinas. Co-administration of MK-801 with NMDA suppressed the effects of NMDA-induced down-regulation of all five genes. Protein levels for NR1(com ), NR2A and NR2B were also monitored at select time points following NMDA-insult. By 2 days-PT, protein levels for the three subunits were dramatically reduced. By day 10, the levels of protein expression for NR1(com)and NR2B remained suppressed despite the rise in gene expression for these two subunits, whereas protein for NR2A showed a substantial rise in expression. Of the five genes assayed, NR2A and NR2B showed the greatest reduction in expression following NMDA treatment, suggesting that one or both of these subunit may signal events leading to neuronal cell death in the retina. Conversely, gene expression of the NR2D subunit was least affected by NMDA exposure. In view of the evidence that the NR2D subunit is expressed by rod bipolar cells in the rat and that these neurons do not die following NMDA insult, it appears that inclusion of this subunit into functional receptors may provide protection against NMDA-induced cell death. Although the significance of the transient down-regulation of four out of the five NMDA receptor subunits is still not fully understood, the recovery of expression of these genes by day 10-PT indicates that not all of the NMDA receptive neurons are susceptible to NMDA-induced cell death. The preferential down-regulation of the NR2A and NR2B receptor subunits may implicate these subunits as key players in mediating the excitotoxic signal in the retina and possibly elsewhere in the brain.

Prosaptide exacerbates ischemia-induced behavioral deficits in vivo; an effect that does not involve mitogen-activated protein kinase activation

Prosaposin is a 517 amino acid membrane component and secreted protein(5,7,9) that is proteolytically cleaved to generate the four small glycoproteins; saposins A, B, C and D.(9,13,19) Prosaposin's ability to promote neurite outgrowth(31) and to protect neurons from programmed cell death(28) in vitro, as well as to rescue neurons from ischemia and other damage in vivo(11,12,15,25) implied that prosaposin was neurotrophic/neuroprotectant.(1,7,24,31) The neurotrophic sequence of prosaposin was isolated to smaller peptide fragments termed prosaptides(15,31) within the amino terminal portion of saposin C.(1,6,8,10,17,20,21,28) The proposed use of synthetic prosaptides as peripherally administered neuroprotective and/or neurotrophic therapeutic agents has stemmed from their ability to cross the blood-brain barrier,(27) as well as their reported neurotrophic activity in vitro.(15,23,31) Few studies, however, have attempted to characterize these peptides, presumably due to their reported instability following peripheral administration.(27) With the recent design of a stable 11-mer retro-inverso prosaptide,(15,31) it has become feasible to investigate the pharmacological effects of a stable version of these peptides in the validated rabbit spinal cord ischemia model that has been used extensively in the development of therapeutics to treat ischemic stroke.(4,14,16,18) Our results show not only that prosaptide was not neurotrophic/neuroprotectant in vivo, but rather it worsened ischemia-induced behavioral deficits.

Perinatal brain injury

Perinatal brain damage in the mature fetus is usually brought about by severe intrauterine asphyxia following an acute reduction of the uterine or umbilical circulation. The areas most heavily affected are the parasagittal region of the cerebral cortex and the basal ganglia. The fetus reacts to a severe lack of oxygen with activation of the sympathetic-adrenergic nervous system and a redistribution of cardiac output in favor of the central organs (brain, heart and adrenals). If the asphyxic insult persists, the fetus is unable to maintain circulatory centralization, and the cardiac output and extent of cerebral perfusion fall. Owing to the acute reduction in oxygen supply, oxidative phosphorylation in the brain comes to a standstill. The Na+/K+ pump at the cell membrane has no more energy to maintain the ionic gradients. In the absence of a membrane potential, large amounts of calcium ions flow through the voltage-dependent ion channels, down an extreme extra-/intracellular concentration gradient, into the cell. Current research suggests that the excessive increase in levels of intracellular calcium, so-called calcium overload, leads to cell damage through the activation of proteases, lipases and endonucleases. During ischemia, besides the influx of calcium ions into the cells via voltage-dependent calcium channels, more calcium enters the cells through glutamate-regulated ion channels. Glutamate, an excitatory neurotransmitter, is released from presynaptic vesicles during ischemia following anoxic cell depolarization. The acute lack of cellular energy arising during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to preischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. The inhibition of protein synthesis, therefore, appears to be an early indicator of subsequent neuronal cell death. A second wave of neuronal cell damage occurs during the reperfusion phase. This cell damage is thought to be caused by the postischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may be caused by induction of a kind of cellular suicide programme known as apoptosis. Interestingly, there is increasing evidence from recent clinical studies that perinatal brain damage is closely associated with ascending intrauterine infection before or during birth. However, a major part of this damage is likely to be of hypoxic-ischemic nature due to LPS-induced effects on fetal cerebral circulation. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies with successful results in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of intravenous administration of magnesium or postischemic induction of cerebral hypothermia.

Pharmacologic neuroprotection in experimental spinal cord ischemia: a systematic review

Various surgical procedures may cause temporary interruption of spinal cord blood supply and may result in irreversible ischemic injury and neurological deficits. The cascade of events that leads to neuronal death following ischemia may be amenable to pharmacological manipulations that aim to increase the tolerable duration of ischemia. Many agents have been evaluated in experimental spinal cord ischemia (SCI). In order to investigate whether an agent is available that justifies clinical evaluation, the literature on pharmacological neuroprotection in experimental SCI was systematically reviewed to assess the neuroprotective efficacy of the various agents. In addition, the strength of the evidence for neuroprotection was investigated by analyzing the methodology. The authors used a systematic review to conduct this evaluation. The included studies were analyzed for neuroprotection and methodology. In order to be able to compare the various agents for neuroprotective efficacy, relative risks and confidence intervals were calculated from the data in the results sections. A total of 103 studies were included. Seventy-nine different agents were tested. Only 14 of the agents tested did not afford protection at all. A large variation was observed in the experimental models to produce SCI. This variation limited comparison of the individual agents. In 48 studies involving 31 single agents, the relative risks and confidence intervals could be calculated. An analysis of the methodology revealed poor temperature management and lack of statistical power in the majority of the 103 studies. The results suggest that numerous agents may protect the spinal cord from transient ischemia. However, poor temperature management and lack of statistical power severely weakened the evidence. Consequently, clinical evaluation of pharmacological neuroprotection in surgical procedures that carry a risk of ischemic spinal cord damage is not justified on the basis of this study.

Hypoxia-induced generation of nitric oxide free radicals in cerebral cortex of newborn guinea pigs

Previous studies have shown that brain tissue hypoxia results in increased N-methyl-D-aspartate (NMDA) receptor activation and receptor-mediated increase in intracellular calcium which may activate Ca++-dependent nitric oxide synthase (NOS). The present study tested the hypothesis that tissue hypoxia will induce generation of nitric oxide (NO) free radicals in cerebral cortex of newborn guinea pigs. Nitric oxide free radical generation was assayed by electron spin resonance (ESR) spectroscopy. Ten newborn guinea pigs were assigned to either normoxic (FiO2 = 21%, n = 5) or hypoxic (FiO2 = 7%, n = 5) groups. Prior to exposure, animals were injected subcutaneously with the spin trapping agents diethyldithiocarbamate (DETC, 400 mg/kg), FeSO4.7H2O (40 mg/kg) and sodium citrate (200mg/kg). Pretreated animals were exposed to either 21% or 7% oxygen for 60 min. Cortical tissue was obtained, homogenized and the spin adducts extracted. The difference of spectra between 2.047 and 2.027 gauss represents production of NO free radical. In hypoxic animals, there was a difference (16.75+/-1.70 mm/g dry brain tissue) between the spectra of NO spin adducts identifying a significant increase in NO free radical production. In the normoxic animals, however, there was no difference between the two spectra. We conclude that hypoxia results in Ca2+-dependent NOS mediated increase in NO free radical production in the cerebral cortex of newborn guinea pigs. Since NO free radicals produce peroxynitrite in presence of superoxide radicals that are abundant in the hypoxic tissue, we speculate that hypoxia-induced generation of NO free radical will lead to nitration of a number of cerebral proteins including the NMDA receptor, a potential mechanism of hypoxia-induced modification of the NMDA receptor resulting in neuronal injury.

White matter injury in spinal cord ischemia: protection by AMPA/kainate glutamate receptor antagonism

BACKGROUND AND PURPOSE

Spinal cord ischemia is a serious complication of surgery of the aorta. NMDA receptor activation secondary to ischemia-induced release of glutamate is a major mechanism of neuronal death in gray matter. White matter injury after ischemia results in long-tract dysfunction and disability. The AMPA/kainate receptor mechanism has recently been implicated in white matter injury.

METHODS

We studied the effects of AMPA/kainate receptor blockade on ischemic white matter injury in a rat model of spinal cord ischemia.

RESULTS

Intrathecal administration of an AMPA/kainate antagonist, 6-nitro-7-sulfamoyl-(f)-quinoxaline-2, 3-dione (NBQX), 1 hour before ischemia reduced locomotor deficit, based on the Basso-Beattie-Bresnahan scale (0=total paralysis; 21=normal) (sham: 21+/-0, n=3; saline: 3.7+/-4.5, n=7; NBQX: 12. 7+/-7.0, n=7, P<0.05) 6 weeks after ischemia. Gray matter damage and neuronal loss in the ventral horn were evident after ischemia, but no difference was noted between the saline and NBQX groups. The extent of white matter injury was quantitatively assessed, based on axonal counts, and was significantly less in the NBQX as compared with the saline group in the ventral (sham: 1063+/-44/200x200 microm, n=3; saline: 556+/-104, n=7; NBQX: 883+/-103, n=7), ventrolateral (sham: 1060+/-135, n=3; saline: 411+/-66, n=7; NBQX: 676+/-122, n=7), and corticospinal tract (sham: 3391+/-219, n=3; saline: 318+/-23, n=7; NBQX: 588+/-103, n=7) in the white matter on day 42.

CONCLUSIONS

Results indicate severe white matter injury in the spinal cord after transient ischemia. NBQX, an AMPA/kainate receptor antagonist, reduced ischemia-induced white matter injury and improved locomotor function.

Prevention of ischemic spinal cord injury: comparative effects of magnesium sulfate and riluzole

PURPOSE

Excitotoxic mechanisms have been implicated in the pathophysiology of spinal cord ischemic injury induced by aortic cross-clamping. We investigated the effects of the anti-excitotoxic drugs magnesium sulfate (MgSO(4)) and riluzole in a rabbit model of spinal cord ischemia.

METHOD

The infrarenal aorta of New Zealand albino white rabbits (n = 68) was occluded for 40 minutes. Experimental groups included: a control group, which received only vehicle (n = 17); group A (n = 17), which received riluzole (8 mg/kg) before clamping; group B (n = 17), which received MgSO(4) (100 mg/kg) before clamping; and group C (n = 17), which received riluzole (8 mg/kg) and MgSO(4) (100 mg/kg) before clamping. Five additional rabbits had the same operation, but did not undergo aortic clamping (sham operation). The neurological status of the rabbits was assessed at 24 hours, 48 hours, and then daily for as long as 120 hours by using a modified Tarlov scale. The rabbits were killed at 24 hours (n = 3 per group), 48 hours (n = 4 per group), and 120 hours (n = 10 per group) postoperatively. Spinal cords were harvested for histopathologic and immunohistochemistry examinations for microtubule-associated protein-2 (MAP-2), a cytoskeletal protein specific from neurons.

RESULTS

No major adverse effect was observed with either riluzole or MgSO(4). All control rabbits became severely paraplegic. All riluzole-treated and MgSO(4)-treated animals had a better neurological status than control animals. Typical morphological changes characteristic of neuronal necrosis in the gray matter of control animals was demonstrated by means of the histopathological examination, whereas riluzole or magnesium prevented or attenuated necrotic phenomenons. Moreover, MAP-2 immunoreactivity was completely lost in control rabbits, whereas it was preserved, either completely or partially, in rabbits treated with riluzole or magnesium. Riluzole was more effective than MgSO(4) in preventing paraplegia caused by motor neuron injury (P <.01 ). Riluzole and MgSO(4) had no additive neuroprotective effect.

CONCLUSION

These results demonstrate that riluzole and, to a lesser extent, MgSO(4) may afford significant spinal cord protection in a setting of severe ischemia and may, therefore, be considered for clinical use during "high-risk" operations on the thoracic and thoracoabdominal aorta.

Pathophysiology of perinatal brain damage

Perinatal brain damage in the mature fetus is usually brought about by severe intrauterine asphyxia following an acute reduction of the uterine or umbilical circulation. The areas most heavily affected are the parasagittal region of the cerebral cortex and the basal ganglia. The fetus reacts to a severe lack of oxygen with activation of the sympathetic-adrenergic nervous system and a redistribution of cardiac output in favour of the central organs (brain, heart and adrenals). If the asphyxic insult persists, the fetus is unable to maintain circulatory centralisation, and the cardiac output and extent of cerebral perfusion fall. Owing to the acute reduction in oxygen supply, oxidative phosphorylation in the brain comes to a standstill. The Na(+)/K(+) pump at the cell membrane has no more energy to maintain the ionic gradients. In the absence of a membrane potential, large amounts of calcium ions flow through the voltage-dependent ion channel, down an extreme extra-/intracellular concentration gradient, into the cell. Current research suggests that the excessive increase in levels of intracellular calcium, so-called calcium overload, leads to cell damage through the activation of proteases, lipases and endonucleases. During ischemia, besides the influx of calcium ions into the cells via voltage-dependent calcium channels, more calcium enters the cells through glutamate-regulated ion channels. Glutamate, an excitatory neurotransmitter, is released from presynaptic vesicles during ischemia following anoxic cell depolarisation. The acute lack of cellular energy arising during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to pre-ischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. The inhibition of protein synthesis, therefore, appears to be an early indicator of subsequent neuronal cell death. A second wave of neuronal cell damage occurs during the reperfusion phase. This cell damage is thought to be caused by the post-ischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may be caused by induction of a kind of cellular suicide programme known as apoptosis. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies with successful results in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of i.v. administration of magnesium or post-ischemic induction of cerebral hypothermia.

Use-dependent exaggeration of brain injury: is glutamate involved?

Extreme overreliance on the impaired forelimb following unilateral lesions of the forelimb representation area of the rat sensorimotor cortex (FL-SMC) leads to exaggeration of the initial cortical injury. Glutamate has repeatedly been implicated in the secondary processes leading to neuronal death following traumatic insult, chiefly because of the neuroprotective properties of excitatory amino acid antagonists in a variety of animal models of brain injury. The present study investigated the possibility that NMDA receptor-mediated processes are involved in use-dependent exaggeration of neuronal injury. Rats were fitted with one-sleeved casts that immobilized the intact forelimb for the first 7 days following FL-SMC lesion, a procedure previously shown to result in use-dependent exaggeration of injury and more severe and persistent limb-use deficits. In the present investigation, administration of MK-801 (1 mg/kg ip once daily on alternate days) during the casting period spared neural tissue surrounding the lesion and enhanced functional recovery of the impaired forelimb. These results suggest a role for NMDA receptor-mediated processes in use-dependent exaggeration of injury.

Riluzole prevents ischemic spinal cord injury caused by aortic crossclamping

BACKGROUND

Recent studies support the involvement of glutamate neurotoxicity in the pathophysiology of spinal cord injury induced by aortic crossclamping. We investigated the effects of riluzole, a neuroprotective drug that blocks glutamatergic neurotransmission, in a rabbit model of spinal cord ischemia.

METHODS

The infrarenal aortas of New Zealand White albino rabbits (n = 40) were occluded for 40 minutes. Experimental groups were as follows: sham operation group (n = 5), control group undergoing occlusion but receiving no pharmacologic intervention (n = 10), experimental group A (n = 10) receiving 8 mg/kg riluzole intravenously 30 minutes before ischemia, experimental group B (n = 10) receiving 4 mg/kg riluzole intravenously 30 minutes before ischemia and at the onset of reperfusion, and experimental group C (n = 10) receiving 8 mg/kg riluzole intravenously at the onset of reperfusion. Neurologic status was assessed at 6, 24, and 48 hours after the operation and then daily until the fifth day. All animals were killed at 24, 48, or 120 hours after the operation. Spinal cords were harvested for histopathologic studies, immunohistochemical studies for microtubule-associated protein 2, and search for morphologic features of apoptosis by the terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate-biotin nick-end labeling staining method.

RESULTS

All animals in the control group became paraplegic. Except for 1 rabbit in group C, all riluzole-treated animals had better neurologic function. Luxol fast blue and terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate-biotin nick-end labeling staining methods demonstrated typical morphologic changes characteristic of necrosis and apoptosis in control animals. Riluzole prevented or attenuated ischemia-induced necrosis, apoptosis, and cytoskeletal proteolysis, depending on the dose and the timing of administration.

CONCLUSION

Riluzole may have therapeutic utility during high-risk operations on the thoracoabdominal aorta.

Selective protection of gray and white matter during spinal cord ischemic injury

BACKGROUND

Ischemic injury in the gray matter is associated with excitatory amino acid neurotransmitters (EAA) release, and in the white matter is associated with intracellular sodium accumulation. We investigated the protective effect during spinal ischemia of the EAA antagonist, 2-carboxypiperazinyl-propylphosphonic acid (CPP), and the sodium channel blocker (2,6-dimethylphenylcarbamoylmethyl) triethylammonium bromide (QX).

METHODS

Sprague-Dawley rats were randomized in four groups, received intrathecally 10 microL of saline, CPP, QX, or QX/CPP, and underwent balloon occlusion of the aorta. Proximal pressure was lowered by exsanguination. In the acute protocol, 28 rats were used to calculate the length of occlusion, resulting in paraplegia in 50% of animals (P50). In the chronic study, 60 rats underwent 11' occlusion. The chronic animals were scored daily for 28 days and submitted to cord histology.

RESULTS

The P50 of QX (11'22") and QX/CPP (11'54") were longer than saline (10'39"), suggesting a beneficial effect. Neurologic scores of all treatment groups (p = 0.0001) and histologic scores of CPP (p = 0.003) and QX/CPP (p = 0.002) were better than saline.

CONCLUSIONS

Protection of spinal cord during ischemia can be achieved with intrathecal administration of selective agents directed to the gray and white matter.

Pathogenesis and pharmacological strategies for mitigating secondary damage in acute spinal cord injury

OBJECTIVE

Experimental models and clinical observations of acute spinal cord injury (SCI) support the concepts of primary and secondary injury, in which the initial mechanical insult is succeeded by a series of deleterious events that promote progressive tissue damage and ischemia. Whereas the primary injury is fated by the circumstances of the trauma, the outcome of the secondary injury may be amenable to therapeutic modulation. This article reviews the pathogenetic determinants of these two phases of injury and summarizes the pharmacological manipulations that may restore neurological function after SCI.

METHODS

Experimental models of SCI and their inherent limitations in simulating human SCI are surveyed. The pathogenesis of primary and secondary injury, as well as the theoretical bases of neurological recovery, are examined in detail. The effects of glucocorticoids, lazeroids, gangliosides, opiate antagonists, calcium channel blockers, glutamate receptor antagonists, antioxidants, free radical scavengers, and other pharmacological agents in both animal models and human trials are summarized. Practical limitations to inducing neural regeneration are also addressed.

RESULTS

The molecular events that mediate the pathogenesis of SCI are logical targets for pharmacological manipulation and include glutamate accumulation, aberrant calcium fluxes, free radical formation, lipid peroxidation, and generation of arachidonic acid metabolites. Enhancement of neural regeneration and plasticity comprise other possible strategies.

CONCLUSION

Pharmacological agents must be given within a narrow window of opportunity to be effective. Although many therapeutic agents show potential promise in animal models, only methylprednisolone has been shown in large, randomized, double-blinded human studies to enhance the functional recovery of neural elements after acute SCI. Future therapy is likely to involve various combinations of these agents.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.

Selected peptides targeted to the NMDA receptor channel protect neurons from excitotoxic death

Excitotoxic neuronal death, associated with neurodegeneration and stroke, is triggered primarily by massive Ca2+ influx arising from overactivation of glutamate receptor channels of the N-methyl-D-aspartate (NMDA) subtype. To search for channel blockers, synthetic combinatorial libraries were assayed for block of agonist-evoked currents by the human NR1-NR2A NMDA receptor subunits expressed in amphibian oocytes. A set of arginine-rich hexapeptides selectively blocked the NMDA receptor channel with IC50 approximately 100 nM, a potency similar to clinically tolerated blockers such as memantine, and only marginally blocked on non-NMDA glutamate receptors. These peptides prevent neuronal cell death elicited by an excitotoxic insult on hippocampal cultures.

Effects of hyperbaric oxygen on neurologic outcome for cerebral ischemia in rats

OBJECTIVE

To evaluate the effect of hyperbaric oxygen (HBO) therapy on neurologic outcome (assessed by the quantal bioassay approach) following acute focal cerebral ischemia in rats.

METHODS

Two separate experimental trials were conducted. Trial 1, a nonblinded experiment, used 38 rats. Trial 2, a blinded experiment, used 59 rats. Focal cerebral ischemia was induced with a surgically placed intraluminal occlusion of the left middle cerebral artery; subsequent removal allowed reperfusion. Arterial occlusion times were varied from 5 to 90 minutes in trial 1, and from 3 to 45 minutes in trial 2. The control groups were maintained at ambient pressure for the duration of each trial. The trial 1 treatment group received a single 30-minute HBO treatment at 2.0 atmospheres absolute (ATA) on the initial day of ischemia. The trial 2 treatment group received 30-minute HBO treatments at 2.0 ATA daily for 4 consecutive days. The animals underwent daily 5-point neurologic examinations. A computerized quantal bioassay was used to determine the ET50--the occlusion time required to cause a neurologic abnormality in half of the animals. The control and treatment ET50 values were compared in each trial using a 2-tailed t-test. An increased ET50 for the treatment vs the control group implied a beneficial effect of HBO; a decrease, the opposite. The study had a power of 80% to detect a difference of 11.4 minutes in the ET50 for a 2-sided alpha = 0.05.

RESULTS

For trial 1: the HBO ET50 was 18.1 +/- 21.9 minutes and the control ET50 was 22.8 +/- 25.0 minutes (p > 0.2). For trial 2: the HBO ET50 was 9.49 +/- 17.4 minutes and the control ET50 was 14.9 +/- 14.2 minutes (p < 0.2).

CONCLUSION

HBO therapy showed no apparent benefit in a rat model as a treatment modality for acute cerebral ischemia with reperfusion.

High dose baclofen is neuroprotective but also causes intracerebral hemorrhage: a quantal bioassay study using the intraluminal suture occlusion method

Agonists of the GABA-A receptor are neuroprotective after experimental stroke, but studies of GABA-B agonists have contradicted each other. To further investigate whether GABA-B agonists may be neuroprotective, we devised a quantal bioassay using the intraluminal occlusion method of inducing reversible cerebral ischemia. Subjects underwent middle cerebral artery occlusion for varying amounts of time, ranging from 5 to 90 min. Behavioral outcome was measured 48 h later with a quantal observational scale: score of abnormal given for any one of asymmetric forepaw flexion on tail lift, asymmetric grip, circling, reduced exploration, seizures, or death. To the grouped response data the logistic equation was used to find the ED50, the duration of occlusion that caused one-half of the subjects to be abnormal. To find the potency ratio for each drug, we divided the ED50 for treatment by that for vehicle. We administered baclofen, a GABA-B agonist, intraperitoneally 5 min after the onset ofischemia. Baclofen (20 mg/kg) was neuroprotective (potency ratio of 3.0, P < 0.05), but a lower dose (10 mg/kg) was not. However, both doses of baclofen caused significantly more intracerebral hemorrhages than control. In awake animals, both baclofen doses caused significant increases in mean arterial pressure, but no changes in other cardiorespiratory variables. The glutamate antagonist MK-801, the GABA-A agonist muscimol, and hypothermia were all protective using the bioassay (potency ratios ranging from 1.5 to 3.0). We conclude that although baclofen (20 mg/kg) may be neuroprotective, its utility is complicated by postischemic hypertension and cerebral hemorrhages.

No effect of glutamate on metabolic disturbances in hippocampal slices of mature fetal guinea pigs after transient in vitro ischemia

The involvement of glutamate in the development of cerebral metabolic disturbances in mature fetuses after transient ischemia was studied using a hippocampal slice model. We investigated the effects of exogenously applied glutamate or glutamate antagonists on the recovery of energy metabolism and protein synthesis rate (PSR) in hippocampal slices of mature guinea pigs after in vitro ischemia. The slices were incubated in a thermostatically controlled flow-through chamber and gassed with carbogen (95% O2/5% CO2). In vitro ischemia was induced by transferring the slices to an aglycemic, artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. In a first set of experiments slices were exposed to 10 mM glutamate during a 20-40 min period of in vitro ischemia. In a second set slices were incubated in aCSF containing MK-801 (100 microM) or kynurenic acid (0.5 mM) 30 min before, during and 2 h after in vitro ischemia. After a 12 h recovery phase, the concentrations of adenylates in the slices were measured by HPLC after extraction with perchloric acid. PSR was calculated from the rate of incorporation of [14C]leucine into tissue proteins. Neither glutamate nor glutamate antagonists had any effect on the postischemic recovery of energy metabolism and PSR when applied during in vitro ischemia. It is therefore concluded that glutamate does not play a major role in the development of metabolic disturbances in hippocampal slices from mature guinea pig fetuses subjected to transient in vitro ischemia.

Effects of the novel NMDA antagonists CP-98,113, CP-101,581 and CP-101,606 on cognitive function and regional cerebral edema following experimental brain injury in the rat

The present study evaluated the effects of two novel N-methyl-D-aspartate (NMDA) receptor blockers and ifenprodil derivatives, CP-101,606 and CP-101,581, and their racemic mixture CP-98,113, on spatial memory and regional cerebral edema following experimental fluid-percussion (FP) brain injury in the rat (n = 66). Fifteen minutes after brain injury (2.5 atm), animals received either (1) CP-98,113 (5 mg/kg, i.p., n = 11), (2) CP-101,581 (5 mg/kg, i.p., n = 13), (3) CP-101,606 (6.5 mg/kg, i.p., n = 12), or (4) DMSO vehicle (equal volume, n = 12); followed by a continuous 24-h subcutaneous infusion of drug at a rate of 1.5 mg/kg/h by means of miniature osmotic (Alzet) pumps implanted subcutaneously. Control (uninjured) animals were subjected to identical anesthesia and surgery without injury and received DMSO vehicle (n = 8); CP-98,113 (5 mg/kg, i.p., n = 3); CP-101,581 (5 mg/kg, i.p., n = 3); or CP-101,606 (6.5 mg/kg, i.p., n = 3). FP brain injury produced a significant cognitive impairment assessed at 2 days postinjury using a well-characterized testing paradigm of visuospatial memory in the Morris Water Maze (MWM) (p < 0.001). Administration of either CP-98,113, CP-101,581, or CP-101,606 had no effect on sham (uninjured) animals, but significant attenuated spatial memory impairment assessed at 2 days postinjury (p = 0.004, p = 0.02, or p = 0.02, respectively). Administration of CP-89,113 but not CP-101,581 or CP-101,606 significantly reduced the extent of regional cerebral edema in the cortex adjacent to the site of injury (p < 0.05) and in the ipsilateral hippocampus (p < 0.05) and thalamus (p < 0.05). These results suggest that excitatory neurotransmission may play a pivotal role in the pathogenesis of memory dysfunction following traumatic brain injury (TBI) and that blockade of the NMDA receptor may significantly attenuate cognitive deficits associated with TBI.

Medical therapy for intracerebral hematoma with the gamma-aminobutyric acid-A agonist muscimol

BACKGROUND AND PURPOSE

No therapy has been rigorously proven effective for intracerebral hematoma, although surgery is frequently used for some types of lobar hemorrhages. Since intracerebral mass causes significant ischemia in surrounding brain, we reasoned that anti-ischemia therapy might improve outcome after experimental hematoma.

METHODS

We stereotaxically injected varying doses of bacterial collagenase into the caudate nucleus of rats. Four hours later we administered intravenously 2 mg/kg muscimol, a potent agonist of the gamma-aminobutyric acid-A receptor (n = 20); 1 mg/kg MK-801, an antagonist of the N-methyl-D-aspartate receptor (n = 17); or saline (n = 28). Forty-eight hours after collagenase injection we rated each animal using a standard rodent neurological examination. The ratings were compared with the amounts of injected collagenase by the quantal bioassay procedure. Brains were then prepared for histomorphometry and brain volumes estimated.

RESULTS

We found that the ED50 for collagenase (amount of enzyme that renders 50% of the subjects abnormal) was 0.77 +/- 0.09 U in saline-treated subjects. Treatment with muscimol significantly increased the ED50 to 1.2 +/- 0.21 U, for a potency ratio of 1.55 +/- 0.34 (t = 1.7, P < .05). MK-801 did not affect outcome. Volume of hematoma was significantly correlated with amount of injected collagenase (n = 33, r = .64, P < .001). Volumes of basal ganglia and white matter were significantly reduced by hemorrhage, and muscimol partially ameliorated this.

CONCLUSIONS

We conclude that muscimol significantly improves neurological outcome after intracerebral hematoma.

No protective effect of the NMDA antagonist memantine in experimental spinal cord injuries

We have investigated the effect of memantine, a clinically used NMDA receptor antagonist, in two experimental animals models of spinal cord injury. The lesions were laser-induced photothrombosis to induce focal spinal cord ischemia and clip compression to mimic traumatic spinal cord injury. Pre- or posttreatment of rats with a dose of memantine (20 mg/kg ip) previously shown to be neuroprotective in cerebral ischemia, failed to affect both the neurological and morphological outcome of ischemic spinal cord injury. Likewise, memantine had no effects on neurological and morphological outcome after experimental traumatic injury. In view of the regional heterogeneity of NMDA receptors, the affinity of memantine for spinal cord NMDA receptors was also determined by studying displacement of [3H] (+)-5-methyl-10,11-dihydro-5-H-dibenzo[a,d]cyclohepten-5-10-imine (MK-801) to rat and human spinal cord homogenates. We found that memantine had an affinity for NMDA receptors in the spinal cord (Ki = 0.58 microM) that was significantly lower compared to that of the cerebral cortex (Ki = 0.23 microM) and that the affinity for NMDA receptors in human spinal cord was even lower. We conclude that in view of available data, memantine should not be chosen for clinical studies on neuroprotection in spinal cord injuries and that the lack of protective effect is most likely due to insufficient affinity of memantine for spinal cord NMDA receptors.